Galactic Leftovers

As you sit peacefully in your easy chair reading this magazine, giant clouds of hydrogen gas are plunging into our galaxy at velocities of several hundred kilometers per second. The thought of these high-velocity clouds may be frightening, even terrifying to you—but rest easy, such hurtling gobs of gas have probably been colliding with our galaxy for many billions of years. Indeed, the Milky Way is surrounded by a complex system of neutral hydrogen gas, which some theories suggest may be left over from our galaxy's formation.

Now some new results suggest that our neighbor, the Andromeda galaxy, also shows evidence of continuing its evolution (The Astrophysical Journal Letters, January 20, 2004). Using the Green Bank radiotelescope in West Virginia, David Thilker of Johns Hopkins University and his colleagues found at least 20 discrete clouds and filaments within 150,000 light-years of the great galaxy. This is the first time that an analogue to the Milky Way's system of high-velocity clouds has been found around another galaxy.

The significance of this network for the continuing formation of the Andromeda galaxy is still not clear. The neutral gas might be the "tip of the iceberg," write the authors, perhaps tracing a much larger amount of ionized gas and even dark matter.—Michael Szpir